The present invention relates to circuit breakers and, more particularly, to magnetic trip assemblies for circuit breakers.
Circuit breakers typically provide instantaneous, short time, and long-time protection against high currents produced by various conditions such as short-circuits, ground faults, overloads, etc. In a circuit breaker, a trip unit is the device that senses current (or other electrical condition) in the protected circuit and responds to high current conditions by tripping (unlatching) the circuit breaker's operating mechanism, which in turn separates the circuit breaker's main current-carrying contacts to stop the flow of electrical current to the protected circuit. Such trip units are required to meet certain standards, e.g., UL/ANSI/IEC, which define trip time curves specifying under what conditions a trip must occur, i.e., short time, long time, instantaneous, or ground fault, all of which are well known.
One type of trip unit used for instantaneous and/or short time overcurrent protection is known as a magnetic trip assembly (magnet assembly). A magnet assembly may be used in conjunction with a thermal trip assembly, such as a bimetallic element, which provides long time overcurrent protection. The combination of the magnetic trip assembly and the thermal trip assembly is commonly referred to as a thermal and magnetic trip unit.
The magnet assembly typically includes a magnet core (yoke) disposed about a current carrying strap, an armature (lever) pivotally disposed on the core, and a spring arranged to bias the armature away from the magnet core. The magnet core is typically U-shaped, with one leg forming a pole face. The armature is typically a planar structure having a flat surface opposing the pole face. Upon the occurrence of a short circuit condition, very high currents pass through the strap. The increased current causes an increase in the magnetic field in the air gap between the pole face and the flat-face of the armature. The magnetic field acts to rapidly draw the armature towards the magnet core, against the bias of the spring. As the armature moves towards the core, the end of the armature moves an associated trip latch, which unlatches the operating mechanism causing the main current-carrying contacts to separate.
While such magnetic trip assemblies work well for high ampere ratings, they may not generate enough force to trip the breaker at lower amperages (e.g., 12.5 times the circuit breaker ampere rating for current ratings 30 amps and above) because of the large air gap inherent in these designs. To overcome this drawback, magnet assemblies including multi-turn coils have been developed to affect a higher magnetic force. Such multi-turn coils are, however, more expensive than the core/armature design.